Plastic extrusion process in chill roll casting of film for improved flatness

ABSTRACT

IN CHILL ROLLING RASTING OF PLASTIC FILM, IMPROVED FLATNESS IS OBTAINED BY OSCILLATING THE INTENSITY OF A FLAT STREAM OF GAS WHICH IS APPLIED TRANSVERSE TO THE DIRECTION OF ECTRUSION. BY ADJUSTMENT OF THE AIR STREAM, THICK OR THIN SPOTS MAY BE REDUCED AND DISTRIBUTED ACROSS THE FILM.

Aug. 3, 1971 A. T. WBDIGER 3,597,515

PLASTIC EXTRUSION PROCESS IN CHILL ROLL CASTING 0F FILM FOR IMPROVED FLATNESS Filed Jan. 22, 1969 INVENTOR. fl/mar 7. Wldfger ted States Pat;

3,597,515 Patented Aug. 3, 1971 115. Cl. 264-89 5 Claims ABSTRACT OF THE DISCLGSURE In chill roll rasting of plastic film, improved flatness is obtained by oscillating the intensity of a flat stream of gas which is applied transverse to the direction of extrusion. By adjustment of the air stream, thick or thin spots may be reduced and distributed across the film.

This invention relates to the extrusion of plastic film, and more particularly relates to an improved method of chill roll casting of synthetic thermoplastic resins.

A substantialy problem exists in the preparation of synthetic thermoplastic film in obtaining what is referred to as a flat film or a lay fiat film. In a chill roll casting process such as is set forth in US. Letters Pat. 3,121,915 wherein a synthetic resinous material is extruded from a slot die and the extruded film passed to a cooled drum or chill roll, the freshly extruded film is stretched between the orifice of the die and the chill roll. The chill roll serves to reduce the temperature of the film and provide control of the degree of stretch introduced between the die and the roll. The cooled film is then removed from the roll and wound onto a supply or mill roll. The film as initially extruded and removed from the chill roll can be flat; that is, readily conformed to a flat surface. However, if minor thickness variations are present in the film which is wound onto the mill roll, film unwound from the mill roll may have sag or bag or other indications of non-flatness due to the cumulative elfect of the minor thickness variations and subsequent shrinkage of the film on the roll. Various methods and means have been employed to eliminate such non-uniformities and reduce them to as small a degree as possible. In the preparation of film by a tube or bubble process, various winding techniques have been employed to distribute the non-uniformities in a random manner across the width of the roll so that the deviation of the unwound film from a flat configuration would be minimal. Employing a chill roll casting process, such an approach is not practical.

It would be desirable if there were available an improved method for the chill roll casting of synthetic resinous film.

It would also be desirable if there were an improved method for reducing non-uniformities in a chill roll cast film.

It would also be beneficial if there were a method which would permit distributing such nonarniformities across the width of a chill roll cast film.

These benefits and other advantages in accordance with the present invention are achieved in a method for the preparation of a synthetic resinous thermoplastic sheet wherein a heat plastified synthetic resinous film forming material is extruded as a linear sheet from a slot die, the sheet being above the thermoplastic temperature of the material, the extruded sheet contacted with a generally arcuate cooled surface, the sheet being maintained against the surface generally adjacent the point of initial contact therewith by means of a generally planiform stream of gas directed toward the arcuate surface and disposed generally transversely to the direction of extrusion, the improvement which comprises cyclically varying the intensity of the planiform stream of gas in a direction generally transverse to the direction of extrusion.

Further features and advantages of the present invention will become more apparent from the following specification taken in connection with the drawing wherein:

The figure schematically depicts apparatus suitable for practicing the method of the invention.

In the figure there is depicted a film extrusion apparatus generally designated by the reference numeral 10. The apparatus 10 comprises in cooperative combination an extruder 11. The extruder 11 is in operative combination with a slot sheeting die 13 having an elongate extrusion orifice 14. A heat plastified thermoplastic resinous sheet 15 emerges from the orifice 14. A chill roll 17 is disposed adjacent the orifice 141. The chill roll 17 has a generally arcuate or cylindrical cooling surface 18 upon which the film 15 is deposited in a generally sheet-like form. The roll 17 rotates in a direction as indicated by the arrow and generally is operated in such a manner that the surface speed is greater than the linear speed of extrusion and the film 15 is stretched or drawn to form a film 15a. An air knife or air doctor 19 is disposed adjacent the roll 17. The air doctor 19 has an elongate discharge orifice 21 which delivers a planiform blast of air generally at the point of contact of the film 15a with the roll 17. The air doctor 19 has a plurality of adjusting means 20 adapted to selectively vary the dimension of the orifice 21 and permit selective adjustment thereof. The air knife 19 is slidably supported on ways 22 which permit lateral oscillation of the air knife in a direction generally normal to the direction of extrusion of the film 15. An air knife actuating means 23 is operatively connected to the air knife 19 and to a frame or support 24. The linear actuator 23 such as an air or hydraulic cylinder serves to oscillate the air knife 19 in the direction indicated by the double headed arrow. Beneficially, the width of the air knife 19 is at least equal to the width of the film 15a plus the distance through which the air knife is moved by the actuator 23. Advantageously, the air knife 19 is twice the width of the film 15a, but considerable benefit is achieved when narrower air knives are employed. The cooled film 15a passes from the chill roll 17 and is wound onto a mill roll 26.

In operation of the apparatus 10 and in the practice of the method of the invention, thermoplastic material is extruded by the extruder 11 through the die 13 from the orifice 141 and onto the rotating chill roll 17 where it engages the arcuate surface 18 and is cooled below its thermoplastic temperature and is subsequently passed to a mill roll such as the mill roll 26. Various tensioning devices, if desired, are readily incorporated between the chill roll 17 and the mill roll 26.. The thickness of the film across the width of the roll is then determined by any convenient measuring means; that is, the transverse film profile. Air is provided to the air knife and in regions where the thickness of the film is greater than desired, the orifice 21 on the air knife is narrowed to reduce the air blast in the particular location. Reduction of the air blast appears to permit the thicker film to stretch to reduce the variation in thickness across the film. The air knife subsequently is then oscillated back and forth across the width of the film in such a manner that the entire width of the film is subjected to the uneven air blast. The uneven air blast shows a strong tendency to cause the nonuniformities to appear at various locations across the width of the film; for example, for the thicker region of the film to move from one edge toward the other edge. Beneficially, oscillation of the air knife delivering a nonuniform blast of gas for a distance as small as 5 percent of the web width improves the flatness of film obtained,

and oscillation of the air knife for 100 percent of the web width very significantly improves the flatness of the film. For many applications, satisfactory adjustment of the air knife is made after die adjustments have been made to obtain as much control as is practically possible of the film thickness.

By observing the windup mill roll, which is the roll 26, thick spots in the film become readily apparent because of the increased diameter of the wound film roll in regions where the film has maximum thickness. Subsequent adjustment of the air knife by closing or reducing the orifice in the region of the film which is the thickest and oscillation of the air knife changes the product from a film exhibiting severe sag or bag to a film having commercially acceptable characteristics. The frequency of oscillation may be varied within wide limits. However, it is usually desirable that the air knife oscillate at least about ten full cycles in the'preparation of a mill roll, and beneficially at least about thirty cycles. For the preparation of most film, an oscillation rate of about one cycle per minute is satisfactory.

Although the process of the invention in one specific embodiment has been described in relation to the oscillation of the air knife, it is obvious that only the cyclic oscillation of thhe variation in intensity in the air stream is necessary to accomplish the purpose of the invention. This can be achieved in a variety of other manners. For example, the adjusting members, such as the adjusting members 20, of an air doctor such as that depicted in the apparatus of the figure can be cyclically varied by means of a rotating cam mechanically coupled thereto or the gap varied by means of variable length adjusting bolts such as are depicted in Us. Letters Patent 2,938,231 wherein the adjusting bolts may be sequentially heated and cooled in accordance with almost any desired pattern by means of a controlled cam operating individual switches to each of the adjusting bolts or varying individual voltage control units. Alternately, a rotating mandrel may be disposed within a fixed air doctor, the mandrel having a helical groove on the surface therof and serving to act as a restrictor to the flow of gas from the plenum of the air doctor to the discharge slot. If, for example, a helically disposed groove having one turn over the length of the rotor is employed, a high pressure zone will traverse the slot for each turn of the rotor; or if a single land equivalent to the groove is disposed on the rotor, a low pressure zone will traverse the air doctor opening. Other means causing a pressure variation to cyclically traverse the air doctor opening will be readily apparent to those skilled in the art.

The present invention is readily employed with a wide variety of synthetic resinous materials and with particular benefit in the preparation of polyolefin films such as polyethylene, polypropylene and resinous film forming polymers of ethylene and/or propylene with other 4 monomers such as ethyl acrylate, acrylic acid and the like.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may difier particularly from those that have been described in the preceding specification and description. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention.

What is claimed is:

1. In a method for the preparation of a synthetic resinous thermoplastic sheet wherein a heat plastified synthetic resinous film forming material is extruded as a linear sheet from a slot die, the sheet being above the thermoplastic temperature of the material, the extruded sheet contacted with a generally arcuate cooled surface, the sheet being maintained against the surface generally adjacent the point of initial contact therewith by means of a generally planiform stream of gas directed toward the arcuate surface and disposed generally transversely to the direction of extrusion, the improvement which comprises cyclically oscilating a variation in intensity in the planiform stream of gas in a direction generally transverse to the direction of extrusion.

2. The method of claim 1 wherein the planiform stream of gas is non-uniform across the width of the sheet.

3. The method of claim 2 wherein the planiform stream of gas is restricted in a location generally equivalent to a portion of the extruded film having a thickness greater than the adjacent area of the film.

4. The method of claim 1 wherein the sheet is subsequently wound onto a roll and the stream of gas is oscillated at a rate sufficient to provide at least 10 full cycles of oscillation during preparation of the roll.

5. The method of claim 1 wherein the sheet is stretched between the extrusion die and the arcuate surface.

References Cited UNITED STATES PATENTS 2,851,733 9/1958 Pangonis et al. 26440 3,159,696 12/1964 Hodgon Jr. 264-93 3,277,227 10/1966 Kesseler et al. 264216 3,341,388 9/1967 Bunyea 264-40 DONALD J. ARNOLD, Primary Examiner G. AUVILLE, Assistant Examiner US. Cl. X.R. 

